(a) Field of the invention
The present invention relates to a vari-focal lens system for cameras, and more specifically a vari-focal lens system for video cameras.
(b) Description of the Prior Art
In the recent years where compact, light-weight video cameras are offered at low prices, demand for domestic video cameras is increasing rapidly. In order to further develop this demand, it is necessary to design more compact video cameras having lower weight and manufacturable at lower cost, thereby posing problems to obtain more compact lens systems having lower weight and manufacturable at lower cost.
Currently, lens systems for domestic video cameras are designed mainly as zoom lens systems specified for a zoom ratio of 6 and an aperture ratio of F/1.2 to F/1.6. This is because these specifications are very advantageous for lens design and adequate for meeting practical needs.
These zoom lens systems generally consist of four lens units as exemplified by the lens systems disclosed by Japanes Unexamined Published Patent Applications Nos. 102208/58 and 153913/58. However, these zoom lens systems have a common defect that each lens system comprises as many as 14 or 15 lens components, requires high manufacturing cost, has a large diameter and is heavy in weight.
Further, there is also known, as a zoom lens system of this type, the lens system disclosed by Japanese Unexamined Published Patent Application No. 110112/61. However, this lens system requires higher manufacturing cost since it adopts extremely strong curvature on a cemented lens surface though it comprises a reduced number of eight lens components in combination with an aspherical surface.
The conventional zoom lens systems consisting of four lens units generally comprise, in the order from the object side, a first lens unit having positive refractive power, kept fixed during variation of focal length and having focusing function, a second lens unit having negative refractive power and movable for varying focal length, a third lens unit shifted for correcting deviation of image point due to variation of focal length, and a fourth lens unit having positive refractive power, kept fixed at all times and adopted for imaging.
This type of zoom lens systems consisting of four lens units are suited for obtaining high vari-focal ratios and high aperture ratios, but generally comprise a very large number of lens components, concretely 14 to 15 lens components in total, since the first lens unit consists of three lens components, the second lens unit consists of three lens components, the third lens unit consists of one or two lens components, and the fourth lens unit consists of six to eight lens components.
In addition to the zoom lens systems consisting of four lens units, there are known zoom lens systems consisting of two lens units. These zoom lens systems comprise, in the order from the object side, a first lens unit having negative refractive power and a second lens unit having positive refractive power, and is so designed as to perform variation of focal length by varying an airspace reserved between these lens units. Such a zoom lens system consisting of two lens units is not suited for obtaining a high vari-focal ratio and a high aperture ratio, but comprises a small number of lens components, concretely six to eight lens components. Further, a stop arranged in the second lens unit is generally shifted together with the second lens unit for varying focal length and focusing of the zoom lens system is performed by shifting the first lens unit frontward.
When this type of zoom lens system consisting of two lens unit is used in a video camera, the mechanism for shifting the stop along the optical axis is made very complicated due to the fact that the stop for a video camera is large and heavy.
As a zoom lens system which has corrected this defect, there is known the zoom lens system disclosed by Japanese Unexamined Published Patent Application No. 266511/62. The official bulletin of this patent discloses a lens system which consists of two lens units and a stop arranged therebetween. However, the patent specification describes power distribution only but provides no actual numerical data clarifying aberration characteristics.